1. Technical Field
The present application relates to a technique of evaluating whether a sound has been heard in comfort. More specifically, the present disclosure relates to a system, processor, method, and computer program of estimating an uncomfortable sound pressure with respect to a sound, in the context of “fitting” of a hearing aid or the like, i.e., adjusting an amount of amplification for each frequency of an external sound to arrive at a sound of a loudness which is appropriate to each individual user.
2. Description of the Related Art
In recent years, people who are suffering from hypacusia because of old age that need hearing aids are increasing. Their number is said to be about 20 million domestically, and about 500 million globally (as investigated by the Japan Hearing Instruments Manufacturers Association). Before beginning use of a hearing aid, “fitting” is required for adjusting the amount of sound amplification for each frequency in accordance with the auditory characteristics of the user. It is generally difficult to complete fitting in one timeg1, and the user needs to visit a hearing aid shop several times to make readjustments. One reason thereof is that an uncomfortable sound pressure (uncomfortable loudness level: UCL) cannot be correctly measured. A UCL is often determined through calculation from a hearing threshold level (HTL) because a test based on subjective reporting would require loud sounds to be made, which leads to psychological stress and fatigue. However, there is a problem in that a calculated UCL which will not reflect individual differences.
“Inner ear auditory testing in patients with normal hearing showing hyperacusis”, Takashi KIMITSUKI et al., Audiology Japan, 2009, Vol. 52, P. 152-156 (Hereinafter, Non-Patent Document 1), discloses a method of actually measuring a UCL through subjective report. A UCL which is actually measured through subjective report may hereinafter be referred to as a subjective UCL. A subjective UCL is obtained by, using an audiometer, presenting a continuous sound to a user by ascending method (i.e., gradually increasing the sound pressure level), and asking the user to report whether it is at a sound pressure that is too loud to be heard for a long time. The sound pressure which is reported by the user is defined as the subjective UCL. “Subjective reporting” involves, after a user hears a sound, the user making a subjective account as to how the sound was felt to him or her.
Moreover, techniques for estimating UCL by using an electroencephalogram, which reflects electrical activities of the brain, are being developed in the recent years. “Estimating uncomfortable loudness levels based on event-related potentials to triplets of auditory stimuli”, Shinobu ADACHI et al., Proceedings of the 51th Japanese Society for Medical and Biological Engineering, O1-10-1, 2012 (Hereinafter, Non-Patent Document 2), discloses a technique of estimating a UCL of a person with normal hearing for each frequency, this technique using an auditory evoked potential in response to triplet sounds of 80 dBHL or less, which is not loud. Through the use of an electroencephalogram after having heard a sound stimulation of a non-loud sound pressure for a short time, a UCL estimation with a high precision is realized in a short time.